Orthogonal frequency division multiplexing (OFDM) has been recently proposed as a multi-access scheme for use in the next generation cellular wireless systems. A critical issue in the design of these systems is the requirement of strict timing synchronization and access control. Although timing synchronization and access control techniques have been extensively studied and many such techniques are known in the art, synchronization and access control in an OFDM multi-user environment presents a number of unique challenges not adequately addressed by conventional techniques.
For example, unlike other multi-user systems, mobiles in OFDM systems generally cannot be synchronized individually. Therefore, in order to maintain in-cell orthogonality and avoid intersymbol interference (ISI), OFDM transmission often requires that all signal paths from all mobiles arrive at a given base station synchronized to within a specified cyclic extension of the base station sampling period. Consequently, OFDM timing synchronization necessitates some form of coordination and feedback amongst the users in a cell.
Additionally, since OFDM synchronization errors must be absorbed into the above-noted cyclic extension, synchronization errors must be much smaller than the symbol period. However, this fractional symbol timing accuracy is difficult to obtain from regular OFDM data symbols. This is due to the fact that OFDM data symbols typically comprise linearly modulated discrete tones whose periods can be as large as the symbol period. Accurately estimating timing from such narrowband signals can be difficult.
A need therefore exists for improved timing synchronization and access control techniques for use in OFDM systems.